Neural tube defects are debilitating birth defects that occur when the developing neural plate fails to close in early gestation. In humans, neural tube defects are characterized by high rates of mortality and lifelong disabilities including paralysis, hydrocephalus and epilepsy. Arsenic induces neural tube defects in animal models, but whether environmental arsenic exposure increases risk of neural tube defects in humans is unknown. Studies investigating arsenic's role in human neural tube defects have been hampered by the relative rarity of these birth defects in the United States and the lack of individual measures of exposure. Though difficult to study, it is critical to understand the role of arsenic exposure in neural tube defects, as exposure to arsenic is common in the United States and throughout the world, and reduction of arsenic exposure and mitigation of arsenic toxicity may present new opportunities for prevention. Our long-range goals are to develop novel screening strategies to identify populations at high risk of neural tube defects and to direct the development of more effective preventive interventions. In this application, we present preliminary data that suggest that folic acid supplementation, currently the primary strategy for neural tube defect prevention, may be less effective in preventing neural tube defects in areas with high arsenic exposure. We now propose a comprehensive set of studies that leverage a unique case ascertainment program in rural Bangladesh, a country that is currently experiencing an epidemic of arsenic poisoning through contaminated drinking water. We plan to understand how arsenic influences risk of neural tube defects in humans through mechanisms that include disruption of maternal glucose and folate metabolism, as well as epigenetic effects. We will also test whether sweat chloride concentration, a potential new biomarker for arsenic toxicity, can be used to identify women at higher risk for having a child affected by neural tube defect. Finally, in an exploratory aim, we propose to collect and study dural tissue, obtained at the time of surgical closure of the defect, to provide insight into the epigenetic mechanisms by which prenatal arsenic exposure affects the developing nervous system. These studies explore mechanisms by which arsenic may increase risk of neural tube defects in humans, and use a unique population with high arsenic exposure in order to test hypotheses. We expect these studies to identify populations at high risk of neural tube defects due to environmental exposures as well as direct development of novel screening strategies for maternal risk. In addition, results from these studies will provie fundamental new knowledge regarding the effect of arsenic on the developing nervous system and the mechanisms that affect developmental neurotoxicity.